In recent years, many RFID tag systems to manage articles using wireless tags (for example, RFID tags) have been proposed. These RFID tag systems transmit response request signals to the wireless tags and receive response signals from the wireless tags through a single communication channel. Accordingly, the RFID tag systems have a problem that a signal collision occurs on the communication channel, which deteriorates the efficiency of ID reading.
In this regard, techniques for improving the efficiency of ID reading of RFID tags are disclosed in Patent Literature 1 to 4. In Patent Literature 1 and 2, a wireless tag reading device (for example, a reader) receives IDs transmitted from a plurality of wireless tags in one time slot which is randomly selected from among a plurality of time slots. Further, in Patent Literature 1 and 2, when the IDs are successfully received, an acknowledge invalidation process for suspending the transmission of signals from the wireless tags is carried out according to a signal output from the reader. According to Patent Literature 1 and 2, the ID reading process and the acknowledge invalidation process are repeatedly executed, thereby enhancing the efficiency of reading IDs from the plurality of wireless tags.
Patent Literature 3 and 4 disclose a technique in which only the wireless tags belonging to the groups which are sequentially set by a reader transmit IDs and the IDs are sequentially read from the wireless tags by limiting the groups as needed depending on the status of occurrence of a signal collision. Thus, in Patent Literature 3 and 4, the reading process is carried out by gradually reducing the probability of occurrence of a signal collision, thereby enhancing the efficiency of the ID reading process.